This application claims the priority of German Application No. 100 37 826.9, filed Aug. 3, 2000 and PCT International Application No. PCT/EP01/08875 filed Aug. 1, 2001, the disclosures of which are expressly incorporated by reference herein.
The invention relates to a method and a device for automatically setting the speed in a vehicle.
The German publication DE 41 00 993 A1 discloses a system for automatically controlling the longitudinal movement of a vehicle and the follow-up travel with respect to a vehicle travelling ahead. According to this system, the distance and the relative speed between the vehicle and the vehicle travelling ahead are determined. The measurement signals are fed to a control unit in which actuation signals, which are fed to actuation elements in the vehicle for automatically setting the vehicle actual speed and the vehicle setpoint speed, are generated from the values of the measurement signals taking into account a predefined conversion relationship.
This system which includes a follow-up control device also permits the vehicle to operate in the stop-and-go mode. In this mode, the following vehicle automatically starts up from a standstill if the vehicle travelling ahead also starts up and the distance between the vehicles is also smaller than a predefined setpoint vehicle distance. The following vehicle accelerates away from a standstill, the vehicle speed of the following vehicle being able to temporarily exceed that of the vehicle travelling ahead until the setpoint distance between the vehicles is re-established. The setpoint distance between the vehicles is also maintained if the vehicle travelling ahead reduces its speed. If the vehicle travelling ahead comes to a standstill, the speed of the following vehicle is also reduced to zero.
This system permits a vehicle to automatically follow a vehicle travelling ahead. In particular, in the lower speed range there is a problem here that frequent starting up from a standstill up to different speeds and renewed braking to a standstill can adversely affect comfort. Furthermore, hazardous situations may arise if in backed-up traffic a relatively high speed is reached briefly in the column of vehicles and in this situation a vehicle moves into the gap between the vehicle which is following and the vehicle travelling ahead, in response to which the system in the vehicle which is following must initiate a comparatively severe braking operation, which entails the risk of rear-end collisions. This problem is also exacerbated by the fact that the speed of the vehicle which is following can briefly exceed the speed of the vehicle travelling ahead so that the vehicle which is following must, in certain circumstances, brake starting from a high speed level.
The invention is based on the problem of improving travelling comfort and travelling safety in vehicles with automatic speed setting.
According to the method in accordance with the invention, a maximum permissible limiting speed is first either predefined or determined from current measurement variables and status variables of the vehicle. If the measurement reveals that the obstaclexe2x80x94for example a vehicle travelling aheadxe2x80x94is moving away from the vehicle, a vehicle setpoint speed which is to be adopted by the vehicle and which the vehicle is not allowed to exceed is determined. The setpoint speed is set to the smaller value of a pair of values formed from the limiting speed and the current speed of the obstaclexe2x80x94in particular of the vehicle travelling ahead. The vehicle actual speed is adjusted to the setpoint speed.
This method ensures that the speed of the vehicle which is following cannot exceed a maximum value, specifically the limiting speed which is to be predefined or determined. This limiting speed cannot be exceeded by the vehicle which is following, even if the speed of the vehicle which is travelling ahead exceeds the limiting speed. In this case, the distance between the vehicle and the vehicle which is travelling ahead increases; but this provides the vehicle occupants of the vehicle which is following with a considerably greater degree of comfort because, in particular in backed-up traffic, short-term acceleration operations and in particular braking operations are eliminated, because the vehicle which is following now follows at an essentially constant speed. Furthermore, safety is improved because vehicles which move into the gap arising between the vehicle which is following and the vehicle travelling ahead no longer give rise to abrupt braking operations in the vehicle which is following because the vehicle which is following is moving at a relatively low speed level.
Maintaining a constant speed when the vehicle travelling ahead adopts a vehicle speed which exceeds the limiting speed results, in backed-up traffic, in a situation in which the vehicle which is following moves at a continuous speed and steadily approaches the end of a column of vehicles which is forming again, and joins on behind the vehicle which is travelling ahead. The steady method of movement avoids the harmonica effect which usually forms in backed-up traffic due to the fact that vehicles are accelerated heavily away from a standstill or from very low speeds and are then abruptly braked again. In addition to improving travelling comfort and increasing travelling safety, this also results in less emission of pollutants.
In a preferred development, the automatic setting of the speed is actuated in a case in which the distance between the vehicle and the vehicle travelling ahead drops below a proximity limit. This criterion can be used, in particular in conjunction with a limiting speed which is set at a low level and which signifies a crawling speed, for triggering the automatic setting of the speed when travelling in backed-up traffic or in heavy town traffic because limiting the vehicle actual speed usually has no negative effects on such journeys because the average speed is comparatively low in any case. The automatic braking of the vehicle to a standstill avoids the risk of accidentally colliding with the obstacle which is travelling ahead.
The method can also be used with obstacles other than vehicles. It is in particular possible to use the method as a parking aid, in which case fixed obstacles can be detected and trigger braking of the vehicle to a standstill.
In a preferred development, it may be specified that after the automatic setting of the speed is activated, the vehicle is automatically braked to a standstill, despite the fact that the obstacle travelling ahead is continuing to move, if the distance between the vehicle and the obstacle drops below a minimum distance. As a result, a minimum safety distance between the vehicle and obstacle is ensured. As soon as the minimum distance is exceeded again, the vehicle is expediently moved off again.
According to one further advantageous embodiment, at least in defined travel situations, if appropriate over the entire activation time of the automatic setting of the speed, a minimum braking torque is generated by means of the vehicle brake in order to achieve the lowest possible vehicle reaction time during braking operations and as a result ensure that the vehicle which is following can be braked in the shortest possible time when the vehicle which is travelling ahead is slowing down or when there is a third vehicle moving into the gap. The minimum braking torque is advantageously generated in all dynamic transition phases of the vehicle which is following, that is to say in particular in acceleration and braking phases. If appropriate, a braking torque can also be applied when the vehicle speed is constant; however, the braking torque is lower than the drive torque when the speed is constant and also in acceleration phases.
The braking torque can, if appropriate, also be at least temporarily reduced, in particular if the distance exceeds a distance threshold, in order to prevent overloading of the brake system.
The automatic setting of the speed is advantageously switched off if the driver expresses a wish, for example by excessively depressing the acceleration pedal, for a vehicle actual speed which exceeds the vehicle setpoint speed. In this case, the driver""s wish has priority over the automatic setting of the speed. If appropriate, it is also possible to take into account a tolerance range which exceeds the setpoint speed and within which the driver can move the vehicle by excessively depressing the acceleration pedal without the automatic setting of the speed being immediately switched off. In this case, the automatic setting of the speed is switched off only when the tolerance range is exceeded. As a result, it is possible that, within the tolerance range, the driver can, by actuating the acceleration pedal, follow a vehicle which is moving quickly ahead while still having the automatic setting of the speed remain activated during a reduction in the depression of the acceleration pedal.
When the system is switched off, any existing braking torque is advantageously not reduced suddenly but rather in accordance with a predefined characteristic curve in order to avoid a sudden jolt and thus possible annoyance of the driver.
The device according to the invention for automatically setting the speed includes a measuring device for determining the distance and the relative speed between the vehicle and an obstacle, in particular a vehicle which is travelling ahead. A control unit, in which actuation signals which can be used to set actuation elements in the vehicle causes the vehicle to automatically approximate the vehicle actual speed to a vehicle setpoint speed which is to be determined from the measurement signals in accordance with a predefined relationship are generated as a function of measurement signals which are recorded in the measuring device. The actuation signals, which act in particular on the braking device in the vehicle, the engine management system and/or the actuation device which regulates the air supply are generated in such a way that the vehicle actual speed is approximated to a setpoint speed which does not exceed the speed of the obstacle which is moving away and is at the same time limited by a limiting speed which is predefined or to be determined. This device for automatically setting the speed in the vehicle is suitable in particular for carrying out the prescribed method with all the method steps.